Multiple-laser pump optical system

ABSTRACT

Apparatus and method for scaling solid-state devices to higher power using multiple sources each of which are separately collimated, followed by focusing of the pump radiation into gain medium colinear to laser mode using a moderated focus. A modularized system is also described.

BACKGROUND OF THE INVENTION

The U.S. Government has non-exclusive rights in this invention pursuantto contract number F19628-85-C-0002 awarded by the Department of the AirForce.

The present invention relates to a method and apparatus for scaling of alaser system to higher power.

End-pumped solid-state lasers are highly efficient, high beam quality,coherent light sources. Typically, end-pumped lasers use a single diodeor a single diode array light source so that the light from such sourcecan be focused into a volume appropriate to the solid-state laser to bepumped. Use of such small scale sources is limited when scaling to highpower, however, by the inability to efficiently focus more than only afew of such devices into the fundamental transverse laser mode.

Power scaling of pumping light sources may be accomplished with varioustechniques. For example, polarization coupling may be used in which twoorthogonally polarized beams from diode lasers or diode laser arrays arecombined at a polarizing beam splitter and then this combined beam isfocused into the solid-state laser medium. Another technique is to usefiber bundles to bring light from many laser diodes, which can bepresented in a nominally colinear manner into the solid-state laseraxis, to achieve end-pumping. In any case, it is desired to increasepump power and to efficiently focus the pump beam into the gain mediumfor a higher power output.

Conventionally, a laser mode is defined by the optics of the opticalresonator in a laser oscillator, or by the beam that is amplified in alaser amplifier.

SUMMARY OF THE INVENTION

The invention includes a method and apparatus for scaling a pumpedmedium to higher power. In one aspect of the invention, a system forpumping a gain medium with multiple lasers includes at least two laserlight sources. The output beam of each light source is substantiallycollimated by respective collimating optics, and the beams of thesources are substantially parallel to each other after collimation.Also, an optical system is provided to focus the collimated and parallelbeams into the gain medium such that the combined beam has a radiusnearly equal to or less than the radius of the fundamental laser modeover a distance on the order of one or more pump absorption lengths ofthe medium. A pump absorption length is the distance required for thepump power to decrease to 1/e of its initial value.

This aspect may include any of the following features: The gain mediumhas an optical resonator which controls the mode radius. The pump laserlight sources are preferably laser diodes or laser diode arrays. Thelight sources are discrete elements. The light sources are laser diodesin monolithic two-dimensional arrays. The beams from the light sourcesare collimated by a monolithic two-dimensional array of lenses. Thebeams from the light sources are collimated by individual lens elements.The collimating optics are designed to compensate for astigmatism in thepump beam. The beams from the light sources are collimated by reflectionfrom curved mirrors. The beams from the light sources are emitted frompump lasers already collimated. The beams from the light sources arecombined in a polarizing beam splitter to be colinear. The opticalsystem contains prisms or cylindrical or spherical lenses for focusingthe collimated beams. The system may further include a multimode opticalfiber into which the combined beam is focused. The gain medium may beNd:YAG, Nd:LiYF₄, Nd:YAlO₃, Nd-doped glass, Nd:YVO₄, Nd:BaY₂ F₈,Nd:GSGG, or other Nd-doped material. The laser may operate on the ⁴F_(3/2) -⁴ I_(11/2), the ⁴ F_(3/2) -⁴ I_(13/2), or the ⁴ F_(3/2) -⁴I_(9/2) transitions of Nd³⁺, or on the ³ H₄ -³ H₆ transition in Tm³⁺,the ⁵ I₇ -⁵ I₈ transition in Ho³⁺, or the ⁴ I_(11/2) -⁴ I_(13/2)transition in Er³⁺, or the ² F_(5/2) -² F_(7/2) transition in Yb³⁺. Thegain medium may be end-pumped or transversely-pumped. The gain mediummay be configured as an amplifier.

In another aspect of the invention, an apparatus for pumping a gainmedium includes a mounting plate having a first station for attachmentof a plurality of excitation modules aligned to have output beams madeparallel to a common axis, and a second station for attachment of a gainmedium whose length is aligned along the common axis. Each of themodules includes a laser light source and associated collimating optics.The source and optics are mounted on a single housing. For a given gainmedium, an optical system is provided to focus the output beams of themodules into a beam having a radius nearly equal to or less than theradius of the laser mode over the length of the gain medium. Apolarizing beam splitter may be used to combine the module outputs in aparallel beam.

In another aspect of the invention, a multiple-laser-pump systemincludes a plurality of excitation sources. The output of each source issubstantially collimated by respective collimating optics. The beams ofthe sources are substantially parallel to each other. An optical systemis provided to focus the substantially collimated and parallel beams toa focal volume. A plurality of optical gain media is provided, each ofwhich is pumped by a respective multiple laser-pump. A combiningstructure is provided for coherently combining the output of the gainmedia into a single beam. The plurality of gain media may be configuredas separate regions in a single monolithic structure.

In another aspect of the invention, a method for increasing powerdensity of pumped radiation to a gain medium includes the steps of: (a)providing a plurality of laser light sources, (b) driving the sourcesand applying the output beam of each to respective collimating optics,(c) applying the output beam of the respective optics to a combiningfocusing optics, and (d) applying the combined output beam of thefocusing optics to a solid-state gain medium such that the beam has aradius about equal to or less than the radius of the laser mode over thelength of the medium. Step (d) preferably includes selection of amoderated focal length where neither the size contribution of thespreading angle of a single beam nor partial overlap of the collimatedbeams dominates the size of the combined output beam in the gain medium.

In another aspect of the invention, a system for coupling light into anoptical fiber includes at least two laser light sources, each lightsource being configured such that each respective output beam issubstantially collimated by respective collimating optics, with thebeams being substantially parallel to each other after collimation, andan optical system configured to focus the collimated and parallel beamsinto a fiber such that the combined beam has a radius nearly equal to orless than the radius of the fiber and a convergence angle nearly equalto or less than the acceptance angle of the optical fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

First we briefly describe the drawings.

FIG. 1 is a schematic representation of a gain medium pumped with amultiple-laser pump in practice of the invention.

FIG. 2 is a schematic representation of a modularized multiple-laserpump in practice of the invention.

FIG. 1 shows a multiple-laser-pumped solid-state system 25 in which amultiple-laser pump 10 pumps a gain medium 28. Pump 10 includes aplurality of laser light sources 12, 14 (such as diode lasers or diodelaser arrays) in a first plane O. Each output beam 20, 22 of source 12,14, respectively, is applied, via respective collimating devices 16, 18,(such as lenses) in a second plane 1 to a focusing assembly 24 in athird plane 2 in order to converge the beams into gain medium 28.

Light sources 12, 14, are separately collimated. After beam collimation,another lens, or set of lenses, or other optics, is used to focus thepump beam into the gain medium. The difference between this techniqueand using a single lens for collimation of all the sources is that inthe latter case, the pump lasers are treated as a single, incoherent,extended source, whereas in the former case, the brightness of eachlight source is maintained while the source outputs are combined toincrease the total pump power.

We have found that to increase the power from the pump source so as toobtain a higher power output from the pumped laser or amplifier, theemitting area of the diode/diode array must be increased, and thisextended beam area then must be focused efficiently into the gain media.The present invention enables such higher power pump beams toefficiently pump gain media to higher power.

We have recognized that if an end pumped gain medium has a length L(which for a given crystal is chosen so that the crystal will absorb agiven fraction of incident pump radiation) and a mode with radius ω_(m)(normally determined by the optical resonator), maximum utilization ofthe pump beam energy will be obtained if the pump beam has a radiusω_(B) which is about equal to or preferably slightly smaller than radiusω_(m) over a distance on the order of one or more pump absorptionlengths of the gain medium. Therefore, for maximum efficiency in scalingto higher power (i.e., increasing the number of light sources in the xand y direction and efficiently capturing their energy for pumping agiven gain medium), beam radius ω_(B) must be minimized down to theconstraint of radius ω_(m) (over length L). Preferably such optimizationis performed by optimally choosing the focusing optics.

We have also recognized the importance of two major factors which effectpump beam size in the gain medium. The first factor is that eachcollimated light source will have a spreading angle. The second factoris that the collimated beams of multiple sources normally only partiallyoverlap at the input and output faces (although overlapped at thecenter) of the medium. The contribution of the first factor to the pumpbeam size is minimized by decreasing the focal length of the focusingoptics, while the contribution of the second factor (partial overlap) isminimized (overlap is increased) by increasing the focal length. Hence,focusing optimization is achieved on a case by case basis by selecting amoderated focal length where the contribution of neither of thesefactors dominates.

The position of the diode laser or diode laser array relative to thecollimating lens is of importance and effects both the degree ofcollimation and the pointing of the collimated beam into the focalelement. As shown in FIG. 2, we provide modules 40, 42 in order toassume uniform collimation of the outputs of light sources 12, 14. Eachof these modules includes a diode laser or a diode laser array 12, 14prealigned with a collimating lens (or lenses) 16, 18 in a singlepackage 52, 54, and can be attached to a reference plate 44 to assureaccurate alignment of the modules relative to each other and to thefocal optics. This avoids the difficulty of trying to align many diodeswith many collimating lenses, and of aligning these combinations intothe focal optics.

After collimation, and before the focal system, all the beams should besubstantially parallel and the far field diffraction angle should beadjusted to a minimum. Then, after focusing, the far field angle (aswell as beam overlap) is adjusted as discussed above to achieve amoderated focus.

In one power scaling experiment, three 500 mW linear diode laser arrays,available from Spectra-Diode Laboratories as model number SDL-2430, wereused to pump a single Nd:YAG laser operated in the fundamentaltransverse mode. The SDL-2430 is a ten stripe array, each stripe being6μ wide and spaced 10μ apart center to center, and with an array outputwavelength of 810 nm. The beam output of these arrays isdiffraction-limited in the y-direction.

The product of the radius ω of the pump beam at a given plane and itsfar field diffraction angle Θ, in a medium having index of refraction n,is expressed by the equivalency ω_(qy) Θ_(qy) n=λ/π for a diffractionlimited beam. The subscript qy (or qx) indicates a quantity at the qthplane in the y (or x) direction. λ is the wavelength of the arrayoutput.

We can use this equivalency to compare the array output beam in the xand y directions. The product of ωΘn for the 2430 array is calculated tobe approximately 7.5×10⁻⁴ cm in the x direction and approximately2.6×10⁻⁵ cm in the y direction (where n=1). Hence, the limiting factorin focusing the beam from the SDL-2430 array into an end-pumped laser isthe beam quality (i.e., the ωΘ product) in the x-direction.

In our experiment, with the three arrays spaced one centimeter apart inthe y-direction, the beams are collimated such that, in the plane of thecollimating lenses, ω_(1y) is calculated at 0.15 cm, and by brightnessconservation, Θ_(1y) is calculated at 1.7×10⁻⁴ rad. But the threecollimated pump beams (one from each array) can be treated as one largepump beam with radius ω' and divergence Θ'. Hence, in this case, in thenon-diffraction-limited x-direction, ω_(1x) =ω'_(1x), Θ_(1x) =Θ'_(1x),and ω'_(1x) Θ'_(1x) =7.5×10⁻⁴ cm; but in the y-direction, Θ_(1y)=Θ'_(1y), and ω'_(1y) is calculated at 1.15 cm, and so ω'_(1y) Θ'_(1y)is about 2×10⁻⁴ cm. With three diodes only we still have ω'_(1y) Θ'_(1y)<ω'_(1x) Θ'_(1x) ; thus, in this experiment, we are able to go furtherand align a total of 10 diode arrays in the y-direction before thebrightness in this direction is about equivalent to that in thex-direction.

We prefer to use off-the-shelf light sources for scaling. Generally, wechoose the larger ωΘ product direction (the x direction in the aboveexperiment) as a target dimension, and then we build the narrowerdirection (y) up to the target dimension (such as by adding additionalarrays in that narrower direction) to generate a beam which optimallyfills the gain medium mode profile (e.g., by forming a roundcross-section beam to fill a round cross-section laser mode), while atthe same time we adjust the focal optics to achieve a moderated focallength (to optimally fill the mode volume). In this manner, we canefficiently scale a system to higher power (i.e., by increasing thenumber of inexpensive--off-the-shelf--light sources) with the focusedbeam radius ω_(B) maintained about equal to or preferably smaller thanthe gain medium radius ω_(m) over the medium length L to optimize ourpumping.

If the above constraints are observed, a practical benefit in scalingoccurs. The area increase in the light source (e.g., by increasing thenumber of light sources in the x and/or y direction) requires only asquare root increase in the laser mode cross-sectional area. Thus, pumppower per unit area at the gain medium increases with scaling. Suchhigher gain is of particular benefit for scaling low gain media.

In the foregoing experiment, we used 0.4 cm focal length lenses tocollimate the beams from the individual arrays. In focusing thecollimated beams into the gain medium, a 15 cm focal length cylindricallens was used to focus the pump beam in the plane that is perpendicularto the junction of the arrays. This provided near optimum focusing (asdescribed above); ω_(B) was slightly smaller than ω_(m) which was ˜220μm over the 0.7 cm length of the gain medium (Nd:YAG). In the orthogonalplane (plane of the junction) a 3.8 cm focal length cylindrical lens wasused to focus the beam to a spot smaller than ω_(m) over the length ofthe gain medium.

Just as the present invention permits scaling of light sources into again medium, it also facilitates scaling of the output power of amultiplicity of end-pumped solid-state lasers by coherent combining.

Lasers which may be used in practice of the invention may include, butare not limited to, for example, lasers operating on the ⁴ F_(3/2) -⁴I_(11/2), the ⁴ F_(3/2) -⁴ I_(13/2), or the ⁴ F_(3/2) -⁴ I_(9/2)transitions of Nd³⁺ or on the ³ H₄ -³ H₆ transition in Tm³⁺, the ⁵ I₇ -⁵I₈ transition in Ho³⁺, or the ⁴ I_(11/2) -⁴ I_(13/2) transition in Er³⁺.This technique is particularly useful for pumping of solid-state lasersthat require high pump intensity, for example, the ⁴ F_(3/2) -⁴ I_(9/2)Nd³⁺, ⁵ I₇ -⁵ I₈ Ho³⁺, the 2 μm ³ H₄ -³ H₆ Tm³⁺ and the ⁴ I_(11/2) -⁴I_(13/2) Er³⁺ transitions.

The present invention may also be applied to transversely pumped gainmedia. Transverse pumping is achieved by applying the pump outputorthogonally to the direction of the output beam. As a result, focusingis mainly required in one direction only. This can be achieved, forexample, with a cylindrical collimating and focusing lens. If high pumpfluence is still needed, 2-d focusing may be used.

In practice of the present invention, it is now possible to use multiplelaser pumps with gain media for various results. It may be used toproduce higher power diode-pumped solid-state lasers, such as a threemicron Er: LiYF₄ laser for medical applications. A diode-pumped, Qswitched Nd:YAG laser can also be produced with higher pulse energies,usable for remote sensing (range finding, etc.), as well as link blowingin semiconductor fabrication, for example. The invention might alsoenable production of a two micron holmium or thulium (safe to the eye)laser for coherent laser radar for airborne wind detection. Otherpossibilities abound.

The gain media may be Nd:YAG, Nd:LiYF₄, Nd:YAlO₃, Nd-doped glass,Nd:YVO₄, or other Nd-doped material, for example. Other materials mayalso be used.

The present invention can also be used to efficiently couple light frommany lasers into an optical fiber. These fibers can be characterized bythe fiber core radius and an acceptance angle (given by the numericalaperture of the fiber for the input radiation). If the ωΘ product of thepump beam from the multiple laser source is on the order of or less thanthe product of the radius and acceptance angle of the fiber, then it ispossible to couple the pump beam efficiently into the fiber.

Furthermore, we can optimize the focusing of the pump beam into thefiber. At the fiber entrance the spot should be on the order of orsmaller than the core size and the convergence angle should be on theorder of or smaller than the acceptance angle of the fiber for efficientcoupling. The output of the fiber can then be used to either pump a gainmedium or for other applications that require high power from the outputof a fiber.

The present invention is not limited to a particular optical package orconfiguration. There may be various optics either before or after thecollimating lens or before or after the focusing lens (or lenses) foradditional beam shaping or for adjustment of beam pointing. For example,the pump beam at plane 1 of FIG. 1 is long in the y-direction and narrowin the x-direction. It may be desirable to focus this beam into the gainmedium with cylindrical lenses or to have a prism pair before thefocusing lens to equalize the beam dimensions. Also, the beam pointingof each collimated diode array could be adjusted with Risley prisms, forexample.

Other embodiments are within the following claims.

We claim:
 1. A system for coupling light into an optical fibercomprisinga multi-beam laser light source for generating a group ofcollimated, substantially parallel, non-coaxial output beams, whereinthe product of the far field divergence angle of the group of outputbeams and the radius of the group of output beams is on the order of orless than the product of the radius of the fiber and the acceptanceangle of the fiber; and an optical system configured to focus thecollimated and parallel output beams into the fiber.
 2. The system ofclaim 1 wherein the optical system is configured to focus the group ofcollimated, substantially parallel, non-coaxial output beams into thefiber as a focused combined beam, wherein the focused combined beam hasa radius at a point of entry into the fiber nearly equal to or less thanthe radius of the fiber and a convergence angle that is nearly equal toor less than the acceptance angle of the fiber.
 3. The system of claim 1wherein said multi-beam light source comprises:at least two laserelements, each laser element being configured to emit a laser beam, andan optical array including for each of said laser elements acorresponding optical element for collimating the laser beam from thatlaser element to produce a corresponding one of said collimated outputbeams.
 4. The system of claim 3 wherein the laser elements are laserdiodes, or laser diode arrays.
 5. The system of claim 4 wherein thelaser elements are discrete elements.
 6. The system of claim 3 whereinthe laser elements are laser diodes in monolithic two-dimensionalarrays.
 7. The system of claim 6 wherein said optical array is amonolithic two-dimensional array of lenses.
 8. The system of claim 3wherein the optical elements of said optical array are individual lenselements within said optical array.
 9. The system of claim 1 whereinsaid optical system comprises a focusing lens.
 10. The system of claim 9wherein said optical system is a focusing lens.
 11. An apparatuscomprising:an optical fiber; a multi-beam laser light source forgenerating a group of collimated, substantially parallel, non-coaxialoutput beams, wherein the product of the far field divergence angle ofthe group of output beams and the radius of the group of output beams ison the order of or less than the product of the radius of the fiber andthe acceptance angle of the fiber; and an optical system configured tofocus the collimated and parallel output beams into the fiber.
 12. Thesystem of claim 11 wherein the optical system is configured to focus thegroup of collimated, substantially parallel, non-coaxial output beamsinto the fiber as a focused combined beam, wherein the focused combinedbeam has a radius at a point of entry into the fiber nearly equal to orless than the radius of the fiber and a convergence angle that is nearlyequal to or less than the acceptance angle of the fiber.
 13. The systemof claim 12 wherein said multi-beam light source comprises:at least twolaser elements, each laser element being configured to emit a laserbeam, and an optical array including for each of said laser elements acorresponding optical element for collimating the laser beam from thatlaser element to produce a corresponding one of said collimated outputbeams.
 14. The system of claim 13 wherein the laser elements are laserdiodes, or laser diode arrays.
 15. The system of claim 13 wherein thelaser elements are discrete elements.
 16. The system of claim 13 whereinthe laser elements are laser diodes in monolithic two-dimensionalarrays.
 17. The system of claim 13 wherein said optical array is amonolithic two-dimensional array of lenses.
 18. The system of claim 17wherein the optical elements of said optical array are individual lenselements within said optical array.
 19. The system of claim 13 whereinsaid optical system comprises a focusing lens.
 20. A system for couplinglight into an optical fiber comprisinga multi-beam laser light sourcefor generating a plurality of collimated, substantially parallel,non-coaxial output beams, said multi-beam laser light source comprisingat least two laser elements and an optical array, wherein each laserelement emits a corresponding laser beam and said optical array includesfor each of said laser elements an optical element that collimates thelaser beam from that laser element to produce a corresponding one ofsaid plurality of collimated output beams, wherein the product of thefar field divergence angle of the plurality of collimated, substantiallyparallel output beams and the radius of the plurality of collimated,substantially parallel output beams is on the order of or less than theproduct of the radius of the fiber and the acceptance angle of thefiber; and a focusing lens that focuses the group of collimated,substantially parallel output beams into the fiber as a focused combinedbeam, wherein said focused combined beam has a radius at a point ofentry into the fiber that is nearly equal to or less than the radius ofthe fiber and a convergence angle that is nearly equal to or less thanthe acceptance angle of the fiber.